The present invention relates to the art of dynamically adjusting print quality. More specifically, the present invention relates to the art of dynamically adjusting number-density of printed dots to control lightness/darkness and/or color.
For printers that print with dots, number-density of dots offers an important control variable in determining print quality. The printer can vary the number density of dots to produce a range of color values (lightness/darkness values) for a particular color ink. Size, dot-gain and optical density of ink dots are typically uncontrolled variables but can be compensated for by adjusting number-density of dots. The color of the medium can be considered an uncontrolled variable as well, but if its variation is known (by user determination or direct measurement), it can be somewhat compensated for in a like manner. Preferably, the printer is calibrated to print nothing to achieve light areas while sufficiently covering areas that are to be dark. A light area is defined here to mean a region on an output medium, for example generally white paper, on which no coverage is desired for a particular colored ink. A dark area is a region on the output medium where an effect of complete coverage is desired using the particular colored ink.
To present what appears to the user as a completely covered area on the output medium, the area need not be completely covered physically with ink. This is because human eyes cannot distinguish color densities beyond a determinable threshold. Thus, for a sufficiently complete coverage to achieve a dark area, dot density doesn't need to be beyond the determinable threshold we may refer to as the visual dark threshold. However, if the visual dark threshold is not reached for a dark area, then the dark area does not appear completely covered, and the print quality suffers. On one hand, if the number-density of dots for coverage of the dark area is less than the visual dark threshold, then the print quality is reduced. On the other hand, if the number-density of dots for coverage of the dark area is greater than the visual dark threshold, then ink or toner has been wasted on that area, and the printing time has been unnecessarily increased. Accordingly, the printer must be calibrated to print its dark regions at the number-density of dots corresponding to the visual dark threshold. This is even more important for color printing where dots of different fundamental colors are used together to produce a desired color output. If the dot number-density of any one of the fundamental colors is off calibration, then the resultant color may not be perceived as the desired color.
Traditionally, calibration of printers regarding number-density of dots is done prior to or during the manufacturing process. Once the printer is shipped, the printer is rarely, if ever, recalibrated. Such calibration may not account for the aforementioned uncontrolled variables or for specific calibration requirements of each individual printer. Moreover, number-density requirement(s) for dot number-densities to achieve a specified value of color (hue, value, chroma, etc.) are different depending upon the color of the medium onto which the printer places its ink, actual dot sizes, dot-gains and specifics of the ink being used.
Accordingly, there is a need for a technique and apparatus for dynamic adjustment of number-densities of dots by the printer.